[llvm] [AMDGPU] Add llvm.amdgcn.dead intrinsic (PR #123190)

[Matt Arsenault via llvm-commits]

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@@ -3442,4 +3442,11 @@ def int_amdgcn_addrspacecast_nonnull : DefaultAttrsIntrinsic<
   [llvm_anyptr_ty], [llvm_anyptr_ty],
   [IntrNoMem, IntrSpeculatable]
 >;
+
+/// Make it clear to the backend that this value is really dead. For instance,
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arsenm wrote:

So is this just convergent poison? Why isn't it IntrConvergent? 

https://github.com/llvm/llvm-project/pull/123190